SNOW MELTING BASED ON DISTRICT HEATING WITH A CASCADE ARRANGEMENT OF SNOW-MELTING UNITS

N. A. POPOV, I. A. KLEMAITIS

(Presented by the Department of Industrial Heat Power Engineering)

In previously published works [1, 2], a substantiation was given of the technical feasibility and economic expediency of melting snow on the basis of district heating instead of hauling it to dumps. It was shown that, during those periods of the heating season when the outdoor-air temperature is above the design temperature for loading the extraction flows of district-heating turbines, and also when daily schedules of ventilation load fail, it is possible, without additional capital investment in the thermal power plant, to increase the generation of electric power on heat consumption by means of snow melting.

The transfer of heat from the thermal power plant for snow-melting purposes through heat networks likewise does not require an increase in pipeline diameters, since snow melting can be carried out during those hours of the day when the load of hot-water supply and ventilation is below its design value. And, finally, the discharge of meltwater from snow-melting units into the existing sewer system (at small hot-water-supply loads) will not cause difficulties.

In the cited works, a case was considered in which the snow-melting units were located at individual heat points (consumer inlets), and the transport of snow to the snow-melting units was carried out by bulldozers.

In view of the fact that the thermal capacity of most consumer inlets is limited, the capacity of the snow-melting units proves insufficient and suitable only for organizing the melting of snow removed from intra-yard areas. When snow is removed from street roadways, where special snow-removal machines are used and the snow is transported by motor vehicles, a considerable increase in the capacity of the snow-melting installations is required.

The latter can be accomplished if the snow-melting units are placed directly along the route of the heat pipelines, connecting them to the pipelines of the interdistrict branches of the heat network according to the scheme shown in Fig. 1.

Snow melting is carried out in vessel 1 by contact between heated meltwater and snow. The circulating meltwater is heated in heat exchanger P-1 by the heat of the network water during those hours of the day when some consumers have no ventilation heat load, and in P-II by the heat of the return water returned to the thermal power plant.

Circulation of the melt water is provided by pump 2, and its spraying by nozzles 3.

The flow of heating water through P-I is produced by the available head between the supply and return lines, and through P-II by transfer pump 4.

Fig. 1. Schematic diagram of a snow-melting installation with a cascade arrangement along the pipeline route of a district heating network

The economic efficiency of snow melting based on district heating, with a cascade arrangement along the pipeline route, is also determined by the payback period of the initial investment in the snow-melting installation, as was done for snow-melting units located at individual heating substations (IHS), with the only difference that here, when determining the annual operating costs for snow melting, additional costs for hauling snow to the snow-melting units by motor transport are taken into account.

In addition, in carrying out this study, certain refinements were made to the initial and cost data in comparison with those adopted in previously published works.

The payback period of the capital investments in the snow-melting installation is written as

\[ T=\frac{K_{\mathrm{st}}}{S_{\mathrm{v}}-S_{\mathrm{st}}},\ \text{years}, \tag{1} \]

where

\(K_{\mathrm{st}}\) — initial investment in the snow-melting installation, rubles;

\[ K_{\mathrm{st}}=a_{2}+b_{2}G_{\mathrm{st}}, \tag{2} \]

\(a_{2}\) and \(b_{2}\) are coefficients, taken on the basis of design-estimate studies as equal to:

\[ a_{2}=1700\ \text{rubles};\quad b_{2}=136\ \text{rubles}\cdot\text{h/t}; \]

\(G_{\mathrm{st}}\) — capacity of the snow-melting installation, t/h;
\(S_{\mathrm{st}}\) — annual operating costs for snow melting, rubles/year;
\(S_{\mathrm{v}}\) — the same for hauling snow to dumps, rubles/year.

Operating costs by variant are

\[ S_{\mathrm{st}}=(S_{\mathrm{a}}+S_{\mathrm{o}}+S_{\mathrm{e}}+S_{\mathrm{k}}+S_{\mathrm{t}}+S_{\mathrm{tr}}^{\mathrm{p}})G_{\mathrm{s}},\quad \text{rubles/year}; \tag{3} \]

\[ S_{\mathrm{v}}=(S_{\mathrm{tr}}^{\mathrm{v}}+S_{\mathrm{otv}})G_{\mathrm{s}},\quad \text{rubles/year}, \tag{4} \]

where

\(S_{\mathrm{a}}\) — unit costs for depreciation and current repair of the snow-melting installation, rubles/t;
\(S_{\mathrm{o}}\) — the same for servicing the installation, rubles/t;
\(S_{\mathrm{e}}\) — the same for electric power for pumping circulating and heating water, rubles/t;

$S_{\text{k}}$ — likewise, for sewerage of meltwater, rubles/t;

$S_{\text{t}}$ — likewise, for fuel, taking into account its saving in combined generation of electric and thermal energy on the basis of snow melting, rubles/t;

$G_{\text{s}}$ — quantity of snow to be melted at the given snow-melting plant, t/year;

$S_{\text{dump}}$ — specific expenditures for payment for maintenance of dumps, rubles/t, adopted according to data of the city municipal services;

$S_{\text{tr}}^{\text{p}}$, $S_{\text{tr}}^{\text{v}}$ — specific expenditures for payment for motor transport, respectively, when bringing snow to the snow-melting plant and when hauling it to dumps, rubles/t.

The formulas for determining the quantities entering into expressions 1, 2, 3, 4, with the exception of $S_{\text{tr}}^{\text{p}}$, $S_{\text{tr}}^{\text{v}}$, $S_{\text{dump}}$, are given in L-2 and therefore are not presented in the present work.

The formula for determining the annual quantity of snow $G_{\text{s}}$ subject to melting at the given snow-melting plant is also changed somewhat:

\[ G_{\text{s}} = F \cdot h_{\text{s}} \cdot \rho_{\text{s}} \cdot \varphi \cdot 10^{-3}, \ \text{t/year}, \tag{5} \]

where

$F$ — area of the territory served by the given snow-melting plant, m²;

$h_{\text{s}}$ — average depth of snow cover, m. Adopted according to data of the meteorological station;

$\rho_{\text{s}}$ — density of snow, kg/m³;

$\varphi$ — fraction of the fallen snow on the indicated area that is subject to melting (hauling).

The specific expenditures for transporting snow by motor vehicles are determined from the formulas:

\[ S_{\text{tr}}^{\text{p}} = \frac{2 \cdot l_{\text{tr}}^{\text{p}} \cdot z_{\text{m}}} {\omega_{\text{m}}^{\text{p}} \cdot G_{\text{m}}}, \ \text{rubles/t}; \tag{6} \]

\[ S_{\text{tr}}^{\text{v}} = \frac{2 \cdot l_{\text{tr}}^{\text{v}} \cdot z_{\text{m}}} {\omega_{\text{m}}^{\text{v}} \cdot G_{\text{m}}}, \ \text{rubles/t}, \tag{7} \]

where $l_{\text{tr}}^{\text{p}}$, $l_{\text{tr}}^{\text{v}}$ — average distance of bringing snow from the place of removal to the snow-melting plant and to dumps, km;

$\omega_{\text{m}}^{\text{p}}$, $\omega_{\text{m}}^{\text{v}}$ — average speed of motor vehicles when transporting snow to snow-melting plants and to the dump, km/h;

$z_{\text{m}}$ — cost of one hour of dump-truck operation, rubles/h;

$G_{\text{m}}$ — weight of snow loaded into the dump-truck body, t.

The distance to dumps is determined according to the general plan of the city. As for determining the average distance of bringing snow to snow-melting plants $l_{\text{tr}}^{\text{p}}$, it depends on the size and configuration of the area from which the removed snow is brought to the snow-melting plant. Taking this area in the form of a rectangle $F = x \cdot y$ (including the areas of streets and intra-block territories), we obtain

\[ l_{\text{tr}}^{\text{p}} = \frac{x + y}{4}. \]

Under actual conditions \(l_{\mathrm{tr}}^{\mathrm{p}}\) may be greater; therefore, for greater reliability we take it equal to \(l_{\mathrm{tr}}^{\mathrm{p}}=\dfrac{x+y}{3}\).

When comparing the alternatives of melting and hauling snow to dumps, we assume that the cost of scraping up the snow and loading it onto dump trucks is the same in both alternatives and is not taken into account in the calculations. We shall show the comparison of alternatives for the conditions of the city of Tomsk.

Fig. 2. Determination of the average distance for delivering snow to the snow melter

Fig. 3. Payback period of capital investments in a snow-melting installation as a function of its capacity and the distance of hauling snow to dumps

Heat supply is provided by Tomsk GRES-II with initial parameters \(p_1=90\) bar, \(t_1=535^\circ\mathrm{C}\). The substituting CES is the Belovskaya electric power station with \(p_1=130\), \(t_1=565^\circ\mathrm{C}\). In the calculations it is assumed: \(\varphi_{\mathrm{st}}=0.06\); \(P=100\) rubles/month; \(p_{\mathrm{n}}=0.12\); \(T_i=1.15\) h/h; \(S_{\mathrm{k}}=0.05\) rubles/t; \(S_{\mathrm{p}}=0.015\) rubles/t; \(z_{\mathrm{t}}=0.0008\) rubles/MJ; \(z_{\mathrm{e}}=0.0055\) rubles/MJ; \(t_{\mathrm{n}}^{\mathrm{ob}}=-14^\circ\mathrm{C}\); \(\varphi_{\mathrm{t}}=0.8\); \(\varphi=0.5\); \(\rho_{\mathrm{s}}=250\) kg/m\(^3\); \(h_{\mathrm{s}}=0.5\) m; \(w_{\mathrm{m}}^{\mathrm{p}}=15\text{–}20\) km/h; \(w_{\mathrm{m}}^{\mathrm{v}}=30\) km/h; \(G_{\mathrm{m}}=1.25\) t; \(s_{\mathrm{otv}}=0.4\) rubles/t; \(z_{\mathrm{m}}=3.16\) rubles/h.

The value \(s_{\mathrm{p}}\) has been adopted on the basis of design studies. Figure 3 gives the dependence of the payback period of capital investments in a snow melter on the hauling distance and the productivity—

Fig. 4. Cost of snow melting as a function of snow-melter productivity

Fig. 5. Cost of hauling snow to dumps as a function of hauling distance

...of the snow-melting unit’s capacity. It is apparent from the graph that the payback period for capital investments increases sharply as capacity decreases; therefore, making snow-melting units (when arranged in cascade) with a capacity of less than 5 t/h would be irrational.

According to the graph in Fig. 4, the cost of melting snow decreases only slightly when snow-melting capacity is increased above 12–15 t/h. The latter is explained by the fact that, as snow-melting capacity increases, the distance and cost of transporting snow also increase. Consequently, from an economic standpoint it will hardly be rational to build snow-melting units with a capacity of more than 20 t/h. Although in individual cases, depending on specific conditions (large snowdrifts, no possibility of building two snow-melting units instead of one), such a limitation will not be mandatory.

For the specific conditions of the city of Tomsk, where the average distance for hauling snow to dumps is 3–4 km, snow melting based on district heating is of definite interest. Attention should be drawn to the high cost of maintaining dumps, which, according to the graph in Fig. 5, amounts to 0.4 rubles per ton of removed snow. Nevertheless, even if this cost is reduced by a factor of 2–3, snow melting as a whole will always be more advantageous for the city than hauling snow to dumps.

Conclusion

As the investigations carried out in works [1], [2], as well as in the present work, have shown, snow melting based on district heating deserves definite attention. Undoubtedly, this is a new undertaking and will require much time for its introduction and development. But considering that every year ever greater demands are placed on urban amenities, while the chronic shortage of motor transport for hauling away snow gives reason to hope that in the near future the problem of snow melting based on district heating will receive due attention from workers in municipal services and power systems, making it possible to put its development on a practical footing.

At present, it is necessary to refine, on an experimental installation, the design of an industrial installation for melting snow.

References

1. A. V. Nikitin. Snow-Melting Units. Moscow: Publishing House of the Ministry of Communal Services of the RSFSR, 1952.

2. E. Ya. Sokolov. District Heating and Heat Networks. Gosenergoizdat, Moscow–Leningrad, 1963.

3. N. A. Popov. Increasing the load of extraction flows of district-heating turbines on the basis of snow melting. Proceedings of TPI, vol. 205, 1972.

4. N. A. Popov. A techno-economic comparison of melting snow on the basis of district heating with hauling it to dumps. Proceedings of TPI, vol. 205, 1972.